1. Field of the Invention
The invention relates to systems and methods for generating electrical power from light radiation, and specifically from solar radiation.
2. Description of the Prior Art
The increasingly aggravated inadequacy of fossil fuels for energy generation of all types has led to many efforts to tap alternative energy sources. A particularly attractive alternative source is light radiation, and particularly solar radiation, which comprises enormous amounts of easily accessible energy and is largely untapped. Among the most important existing devices for converting solar energy into electricity are devices of the type developed in the space effort. These devices comprise networks of smaller area thin monocrystalline layers connected in series. These devices have relatively high efficiency in terms of power generation in relation to weight. This criterion, however, is substantially inapplicable to the problem of power generation for normal commercial and consumer purposes, in which the criterion of usefulness is related to economic factors, such as power generation per unit cost. Under this criterion of efficiency units which are useful in the space effort are impractical. These units are comprised of photoelectric material which is substantially monocrystalline and must be grown from crystalline solution, with a high failure rate. These constraints limit such units to small dimensions and require many such units to provide even a minimal power source.
Devices and fabrication techniques utilizing polycrystalline semiconductive materials have generally proven inadequate due to high production costs. Among the contributing factors to these high costs is the requirement of use of structural materials of high heat resistance due to the high temperature utilized with these fabrication techniques. Moreover, such devices generally utilize metallic internal conductors, thus further increasing costs. Also, the failure rate in fabricating such devices is relatively high because of penetration by impurities, in the course of fabrication. Further, control of the deposition of semiconductive material in such processes presents substantial problems.
There has been a recent attempt to form photo-voltaic power generating means with a compound semiconductor material having an N-type region and a P-type region and in which the N-type and P-type regions were doped. In this case, the first semiconductor section, constituting an N-type section, was formed by a vapor phase deposition of a metal, such a cadmium with the addition of sulfur to provide a cadmium sulfide layer. The second, or P-type, semiconductor section was formed by dipping the material into a hot aqueous solution of CuCl which caused formation of Cu.sub.x S. However, the results were poor since the cadmium sulfide was sometimes porous, giving rise to shorted junctions. In addition, a large amount of unused cadmium was required in the deposition, thereby creating a substantially expensive photo-voltaic power generating means, oftentimes of low efficiency.
U.S. Pat. No. 3,573,177 to William McNeill describes a prior art technique by which polycrystalline cadmium, zinc, or cadmium-zinc sulfide or selenide is formed by electrochemical deposition on a Cd or Zn or Cd, Zn anode and where sulfur or selenium is provided from a solution containing S.sup.= or Se.sup.= ions and which polycrystalline material is usable as a semiconductor material. The concept of forming thin films of semiconductor materials by electrochemical techniques is relatively new and due, in part, to the teachings in the aforesaid McNeill patent. In accordance with the McNeill patent, electrochemical discharge of ions, such as those yielded by sulfides or selenides dissolved in an electrolyte, occur with respect to cadmium or zinc acting as an anode in an electrolytic cell. This electrochemical discharge converts the zinc or cadmium, or the alloys of these metals, to the corresponding sulfides or sulfoselenides.
The McNeill patent has advanced the art of producing semiconductive materials by electrochemical techniques and presents many advantages, including the ability to apply films of semiconductor materials to irregularly shaped substrates which were not thoroughly cleaned. Nevertheless, the McNeill patent suffers from many limitations in that the end product is not necessarily capable of functioning as a P-N semiconductor junction material necessary in the operation of a photo-voltaic cell or similar diode. McNeill is essentially concerned with the manufacture of non-junction semiconductor films, such as those found in the electroluminescent panels, electrosonic transducers and photosensitive conductors.
The essence of the McNeill patent lies primarily with anodic plating, with discharge of S.sup.= and Se.sup.= ions. However, it is not known that the McNeill process can be applied to e.g. discharge of Te.sup.= ions which is more ideal in the case of photo-voltaic cells. Yet it is such a discharge, forming CdTe and a ZnTe that is of prime importance for the manufacture of solar cells since cadmium telluride has a direct band-gap uniquely optimized for sunlight at 1.5 eV.
There has also been a proposed prior art technique for electrochemically precipitating metals at a cathode for producing a selenium rectifier. This technique is reported in an article entitled, "Electrochemische Abscheidung von Metallseleniden", by H. Von Gobrecht, H. D. Liess and A. Tausend, in Ber. Deutsche Bunsengesellschaft, Vol. 67 (1963), page 930. This article does not describe the production of photo-voltaic power generating cells. In accordance with this prior art technique, deposition of the less noble component and the more noble component must be very carefully controlled due to the difference in standard precipitation potentials. The more noble component had to be added in carefully controlled small doses in order to operate with this technique.
Therefore, there has been, and is, a well recognized, but unfulfilled need for photo-voltaic power generating means having relatively high power generating capability per dollar of cost to produce and having a form suitable for commercial and consumer use, and for a method for producing such means.